1. Field of the Invention
The present invention relates to a fluidized catalytic cracking process to produce petrochemicals such as olefins and aromatics and improved quality distillate product.
2. Description of Related Art
Olefins (i.e., ethylene, propylene, butylene and butadiene) and aromatics (i.e., benzene, toluene and xylene) are basic building blocks which are widely used in the petrochemical and chemical industries. Thermal cracking, or steam pyrolysis, is a major type of process for forming these materials, typically in the presence of steam, and in the absence of oxygen. Feedstocks for steam pyrolysis can include petroleum gases and distillates such as naphtha, kerosene and gas oil. The availability of these feedstocks is usually limited and requires costly and energy-intensive process steps in a crude oil refinery. These compounds are also produced through refinery fluidized catalytic cracking (FCC) process using typical heavy feedstocks such as gas oils or residues. FCC units produce a significant portion of propylene for the global market.
In FCC processes, petroleum derived hydrocarbons such as heavy feedstocks are catalytically cracked with an acidic catalyst maintained in a fluidized state, which is regenerated on a continuous basis. The main product from such processes has generally been gasoline. Other products are also produced in smaller quantities via FCC processes such as liquid petroleum gas and cracked gas oil. When the heavier feed contacts the hot catalyst and is cracked to lighter products, carbonaceous deposits, commonly referred to as coke, form on the catalyst and deactivate it. The deactivated, or spent, catalyst is separated from the cracked products, stripped of removable hydrocarbons and passed to a regeneration vessel where the coke is burned from the catalyst in the presence of air to produce a substantially regenerated catalyst. The combustion products are removed from the vessel as flue gas. The heated regenerated catalyst is then recycled to the reaction zone in the FCC unit. A general description of the FCC process is provided in U.S. Pat. No. 5,372,704, the complete disclosure of which is incorporated herein by reference.
FIG. 1 plots ranges for general types of technology used to upgrade atmospheric residues (350° C.+) from crude oils. Feeds to be converted in the FCC process should satisfy certain criteria in terms of the metals content and the Conradson Carbon Residue (CCR) or Ramsbottom carbon content as seen in FIG. 1. For instance, residual oils have a large percentage of refractory components such as polycyclic aromatics which are difficult to crack and promote coke formation in addition to the coke formed during catalytic cracking reactions. Because of the high Conradson carbon content, the burning load on the regenerator is increased requiring modifications and upgrades. In addition, these feeds can contain large amounts of metals including nickel and vanadium, which rapidly deactivate the FCC catalyst.
Limiting the amount of resid in the FCC feed has been the most common method in controlling regeneration temperature. Consideration has also been given to integrating catalyst coolers and two-stage regenerator systems. Feeds with up to about 3 wt % CCR can be processed in single stage regenerators, increasing to 6-7 wt % CCR in single stage regenerators with catalyst coolers and to about 10-11 wt % CCR with two-stage units with catalyst coolers. Hydrotreating the heavy feeds prior to cracking is also known to overcome these issues, necessitating higher capital costs and make-up hydrogen sources. FIG. 2 shows the distribution of feeds conventionally used within the FCC processes worldwide [SFA Pacific, Phase 8].
Other lighter feedstocks such as olefinic or paraffinic naphtha are also considered as possible FCC feeds to optimize propylene yield. Because of the comparatively low tendency in forming coke necessary for the heat balance of the FCC unit, naphtha co-processing schemes have been proposed with various configurations within a classical FCC process [Catalysis Today 106 (2005) 62-71]. It is known to combine naphtha with the feed and introduce the combined feed through the same injectors, incorporating a naphtha feed via a riser downstream of the feed injection system, injecting a naphtha feed upstream of the feed injectors (where it is cracked at higher temperature and catalyst-to-oil ratio (C/O) than in classical cracking) and integrating a second reaction zone in which a light naphtha fraction is cracked at higher severity levels.
Conventional feedstocks for FCC process are usually available in relatively limited quantity and are derived from costly and energy intensive processing steps within the refinery. To be able to respond to the growing demand of petrochemicals like propylene, other type of feeds which can be made available in larger quantities, such as raw crude oil, are attractive to producers. Using crude oil feeds will minimize or eliminate the likelihood of the refinery being a bottleneck in the production of these necessary petrochemicals.
Converting raw crude oil in conventional petrochemical manufacturing processes is challenging. In the case of FCC processes, a primary concern is the accelerated deactivation of catalyst due to the presence of comparatively high content of metals and coke precursors.
In addition, operating conditions such as temperature can be difficult to define due to the very wide boiling temperature range of a crude oil feed. Crude oil contains different components that have different cracking reactivity. The components found in the lower boiling temperature fractions, e.g. alkanes in the naphtha range, are typically very less reactive than, for instance, alkyl side chains of naphthenes components present in heavier boiling temperature fractions. According to known teachings, operating conditions employed for a comparatively wider range of boiling temperatures in the feed relative to conventional FCC processes minimizes optimal conversion of the different components. This is clearly illustrated by Corma et al. [Applied Catal. A: General 265 (2004) 195] in which a feed composed of 15 wt % light straight run (LSR) naphtha and 85 wt % gas oil was cracked in a micro downer testing unit. At an operating temperature of 550° C., and using a blend of two catalysts including one designed to promote naphtha cracking, the LSR naphtha does not crack but instead acts as diluents for the gas oil and lowers the overall gas oil conversion.
Conventionally known and commercially operable FCC apparatus and processes can employ multiple reactor stages and rely on feedstocks ranging from naphtha and gas oils to residual oils, which can be limited in availability or must undergo costly and energy intensive refinery processing steps. Therefore a need remains in the industry for efficient FCC apparatus and processes that can maximize production of petrochemicals such as light olefins, e.g., propylene, while minimizing or obviating the need for refinery processing steps to prepare the feedstock.